A lactide has been useful as a starting material for producing a polylactic acid which is a biodegradable polymer. Thus, a ring-opening polymerization of a lactide yields a polylactic acid. In addition to the utility as a starting material for producing such polylactic acid, a utility of a lactide as a food additive such as a preservative, a pH regulating agent, a coagulating agent, an acidulant and an auxiliary expanding agent is expected.
A lactide is a dimeric cyclic ester of lactic acid, and exists as three optical-isomers, i.e., L-lactide formed from two L-lactic acid molecules, D-lactide formed from two D-lactic acid molecules and meso-lactide formed from L-lactic acid and D-lactic acid.
A lactide is produced usually by a reaction distillation method in which lactic acid is subjected to a dehydration condensation to obtain a polylactic acid having a relatively low molecular weight as an intermediate, which is then depolymerized and cyclized to form a lactide, which is isolated as a vapor from the system of reaction.
In such production method, a lactide vapor contains not only L-lactide and/or D-lactide and meso-lactide but also some impurities such as lactic acid monomer, a lactic acid polycondensate having a low molecular weight such as linear dimer and trimer of lactic acid and water and the like. In addition, other impurities derived from a starting lactic acid such as saccharides, amino acids and fatty acids other than lactic acid may also be contained.
Accordingly, it is necessary to purify, depending on the intended use, a crude lactide once cooled and recovered whereby reducing the impurities and meso-lactide contained therein as described above.
Thus, while a lactide is hydrolyzed to form lactic acid and a linear lactic acid polycondensate having a low molecular weight, meso-lactide undergoes far more extensive water absorption and hydrolysis when compared with L-lactide and D-lactide. Accordingly, a lactide having a high meso-lactide content undergoes a rapid hydrolysis as a whole, resulting in a higher concentration of acidic components such as lactic acid and a linear lactic acid polycondensate having a low molecular weight.
For example, when a lactide is employed as a food additive, one which gives a low acidity at an early stage after addition followed by a higher acidity at a later stage (in response to the maturing of the food) is preferred. When large amounts of meso-lactide, lactic acid and a linear lactic acid polycondensate having a low molecular weight are contained in a lactide, the acidity at an early stage after addition becomes high. Accordingly, it is preferable that the concentrations of meso-lactide, lactic acid and a linear lactic acid polycondensate having a low molecular weight in a lactide are as low as possible.
On the other hand, the melting points of L-lactide and D-lactide are both approximately 98.degree. C., while that of meso-lactide is approximately 40.degree. C. The melting point of lactic acid is 16 to 25.degree. C., while a linear lactic acid polycondensate having a low molecular weight exists as a liquid at normal temperatures. As a result, a lactide, when used as a powder or a granule in the presence of high concentrations of meso-lactide, lactic acid and a linear lactic acid polycondensate having a low molecular weight, each of which has a melting point not higher than or close to normal temperatures, results in a poor fluidity of the lactide, which may lead to a poor workability. Accordingly, it is preferable that the concentrations of these substances in a lactide are as low as possible.
Also when a lactide is used as a food additive, the impurities contained in the lactide, such as saccharides, amino acids and fatty acids other than lactic acid, may cause deterioration or change in the taste of the food additive or a problematic coloring of the food additive. Therefore, it is preferable that the concentrations of these impurities in a lactide are as low as possible.
Various conventional methods for purifying a lactide are known, including a recrystallization method, a melt crystallization method, a rectification method, a water extraction method and the like.
For example. Japanese Examined Patent Publication No. 51-6673 discloses a method in which a lactide is recrystallized using a amyl alcohol or butylalcohol solvent. However, this method allows an organic solvent which is hazardous to human health to remain in the lactide once purified, thus being problematic when applied to a food additive.
Japanese Laid-Open Patent Publication No. 63-101378 discloses that a lactide is recrystallized from an alcohol having 1 to 6 carbon atoms, preferably from isopropyl alcohol, or is dissolved in such alcohol and then precipitated using a non-solvent. Japanese Laid-Open Patent Publication No. 7-118259 discloses a method in which a lactide is recrystallized from a lower alcohol and then recrystallized from benzene or equivalent. However, any of these methods is disadvantageous economically since it involves problematically extensive time period and operations in a step in which a crude lactide is dissolved in a solvent with heating and in a step in which a lactide is cooled to precipitate from the solution and also involves a large amount of a solvent and a low yield. In addition, each of these methods allows an organic solvent which is hazardous to human health to remain in the lactide once purified, thus being problematic when applied to a food additive.
Japanese Laid-Open Patent Publication No. 6-256340 discloses a method for purifying a crude lactide by a melt crystallization. However, a melt crystallization is disadvantageous economically since it involves the use of an expensive large scale device and also involves problematically extensive time period and operations in a step in which a lactide is cooled to precipitate or in a step in which a heating and a purification/dissolution are conducted. In addition, a step for separating a purified lactide which is conducted at a temperature higher than the melting point of a lactide allows a further degradation of a lactide to take place, thus allowing a problematic formation of lactic acid and a linear lactic acid polycondensate having a low molecular weight to take place. In order to obtain a lactide of a constant quality, a strict process control is required.
Published Japanese Translation Patent Publication No. 7-505150(WO93/18020) discloses a method for obtaining a highly pure lactide by a rectification. However, such rectification requires a complicated and expensive instrument and involves problematically extensive operations in a step in which a lactide is evaporated with heating or in a step in which a lactide is condensed with cooling, thus presenting an economical disadvantage. In addition, a step for separating a purified lactide which is conducted at a temperature higher than the melting point of a lactide allows a further degradation of a lactide to take place, resulting in a problematic formation of lactic acid and a linear lactic acid polycondensate having a low molecular weight. In order to obtain a lactide of a constant quality, a strict process control is required here again.
Japanese Laid-Open Patent Publication No. 7-165753 discloses a method in which a crude lactide is brought into contact with water to effect a purification. In this method, a lactide is brought into contact with water in an attempt to remove meso-lactide, but a sufficient removal of meso-lactide is associated inevitably with the hydrolysis and the removal also of a part of L-lactide and/or D-lactide, resulting in a reduced yield. Subsequently, the removed L-lactide and/or D-lactide undergoes a reaction with water, and can be recovered only in the form of lactic acid or a linear lactic acid polycondensate having a low molecular weight. The fact that the lactide which has been obtained from lactic acid through an extensive operation can be recovered only in the form of lactic acid or a linear lactic acid polycondensate having a low molecular weight means an extremely poor cost efficiency.